Tools for textiles, in particular needles, are typically made of carbon steel and hardened as needed. For example, document DE 199 36 082 A1 discloses a sewing needle and a knitting needle, each consisting of carbon steel. In order to increase the surface hardness, the blank is subjected to a thermal treatment or a shot peening treatment to produce the needle. Consequently, the result is a superficial hardening of the tool for textiles.
Document DE PS 21 14 734 describes a method for tempering hardened needles, wherein longitudinal zones exhibiting differing hardnesses are the result. This is accomplished by supplying different heat quantities to the individual longitudinal zones of the needles. In the case of this method, the size of the hardened zones is decisively determined by the size of the zones that are heated on the needles during the hardening process.
From document U.S. Pat. No. 4,049,430 the hardening of stainless chromium-nickel steel obtained by precipitation hardening is known. The steel consists mostly of a chromium-nickel-copper-aluminum structure, wherein the content of carbon is limited to less than 0.05%. In order to produce the desired hardness, a nickel content of 8.5% to 9.5% is provided. The chromium content is restricted to 11.75% to avoid the formation of ferrite.
In principle, it has also been known to harden chromium-containing steels by carburization. Regarding this, documents WO 2011/017495 A1, as well as U.S. Pat. No. 6,093,303, have provided that the object to be hardened be initially freed of a passive layer of chromium oxide preventing the penetration of carbon entry and be then exposed at relatively low temperatures of less than 540° C. to a carbon-donating low pressure atmosphere. In WO 2011/017495 the carbon-donating gas is acetylene. Both literature references aim to prevent a carbide formation in the steel.
Tools for textiles typically display relatively fine structures that are subjected to various conditions during operation. The so-called working portion is formed, for example, in felting needles by a frontal longitudinal tip with one or more hooks or barbs, in a sewing needle by the eye and by other parts coming into contact with the textile and thread, in a hook needle by the hook and the directly adjacent part of the shank, in a tufting gripper by the lower edge for loop acceptance, and in a knife by the cutting edge. These working portions must be highly wear-resistant and hard, yet still be resistant to fracturing. In contrast, the remaining shank of the tool for textiles should frequently satisfy other requirements. From this, there results the wish not only just for a zone-wise hardening but also the wish for differing hardening depths or hardening gradients in the tool for textiles. For example, in the case of a sewing needle it may be attempted to harden the eye region through and through, whereas the adjoining shank portion not coming into contact with the thread should only be surface-hardened. Consequently, various hardening depths may be desirable at various points of the surface of the tool for textiles. Moreover, hardness gradients in depth direction of the tool for textiles may be desirable at various points of this surface.
Furthermore, the tool for textiles is subject to a large spectrum of conditions of storage and use. It must be storable for extended times at various temperatures and humidities, without losing its properties and without corroding. Quenching and tempering treatments as have been suggested by document DE 199 36 082 A1 are provided for increasing the corrosion resistance. One such quenching and tempering treatment may be, for example, galvanic chromium plating.